Following the treatment of mice with biological response modifiers, we have observed a recruitment of cells with natural killer (NK) activity to the liver and spleen, presumably in response to the local release of cytokines or other mediators. To test whether specific cytokines could stimulate migration of NK cells, we used a modified Boyden chamber assay to study their chemotaxis in vitro. Recombinant human (rh) interleukin (IL)-2, rhIL-6 and rh tumor necrosis factor Alpha (TNFAlpha) induced directed migration of purified rat NK cells in a dose and time-dependent fashion. In contrast, rhIL-1Alpha, rhIL-8, r rat interferon-Gamma (IFN-Gamma) and rhTNFBeta were inactive in this assay, suggesting that regulation of NK recruitment involves responses to specific inflammatory cytokines. Splenic NK cells and the murine vascular endothelial cell line eEND-2 were used to study the effects of cytokines on NK cell adhesion and penetration through capillary endothelial barriers. Treatment of eEND-2 cells with 10-1000 U/ml of rmTNFAlpha resulted in a 4-7 fold increase in NK cell adhesion. rhIL-1 treatment of eEND-2 cells also increased NK cell adhesion 2-4 fold over background levels. In contrast, treatment of eEND-2 with either rhIL-6, rhIL-7 or rmIFN-Gamma had no effect on NK cell adhesion at any of the doses tested. Pretreatment of the NK cells alone with the cytokines produced only a modest increase in their adhesiveness, suggesting that modulation of surface adhesion molecules on eEND-2 cells by rmTNFAlpha is responsible for the enhanced effects. Future studies will focus on the nature of these molecules and their counter-ligands present on NK cells. Taken together, these results suggest that local release of cytokines in tissue sites following BRM treatment may regulate the recruitment and activation of leukocyte subsets in vivo. To test this hypothesis, isolated rat Kupffer cells were treated with various BRM or cytokines and examined for cytolytic activity against rat hepatoma cells. Only treatment with rhIL-2 was capable of activating rat Kupffer cell cytotoxicity directly, suggesting that different signals are required to stimulate leukocyte recruitment and activation.